Filament-current control unit in electron-beam apparatus

ABSTRACT

An electron-beam apparatus is provided with an electrode for intercepting a portion of the electron beam so as to achieve automatio cathode temperature control. The filament current is controlled by means of a signal derived from this interception electrode.

United States Patent [1 1 [451 Feb. 25, 1975 Van Zuijlen et al.

[ FlLAMENT-CURRENT CONTROL UNIT IN ELECTRON-BEAM APPARATUS [75]Inventors: Peter Van Zuijlen; Leendert Antonius Fontijn, both of Delft,Netherlands [73] Assignee: U.S. Philips Corporation, New

York, N.Y.

[22] Filed: June 1, 1972 [21] Appl. No.: 258,549

[30] Foreign Application Priority Data June 5, 1971 Netherlands 7107749[52] US. Cl. 315/94 [51] Int. Cl. HOlj 3/08 [58] Field of Search 315/94,106, 107, 111, 315/30, 31, 307; 219/121 EB, 121 EBM;

l78/DIG. 29

Primary Examiner-Ronald L. Wibert Assistant Examiner-Richard A.Rosenberger Attorney, Agent, or Firm-Frank R. Trifari; George B. Berka[57] ABSTRACT An electron-beam apparatus is provided with an electrodefor intercepting a portion of the electron beam so as to achieveautomatio cathode temperature control. The filament current iscontrolled by means of a signal derived from this interceptionelectrode.

5 Claims, 3 Drawing Figures l 0 wnlnwlln v v v v v v 539292 41 View? 1715 8 am" IATENTEUFEBPS I975 sum-:12 BF 2 Fig.2

FILAMENT-CURRENT CONTROL UNIT IN ELECTRON-BEAM APPARATUS The inventionrelates to an electron-beam apparatus comprising a cathode for thermalemission of an electron beam, a Wehnelt electrode, and afilament-current control unit.

In devices comprising a thermal cathode for generating an electron beamfor electron-optical applications it is of essential importance that thecathode temperature during operation is always optimum for the emissionof an electron beam of this kind. An operating temperature which islower than the optimum temperature results in a comparatively smallemissive power of the cathode. As a result, an electron beam to beemitted by the cathode has, at least on the cathode surface, acomparatively small current density. This has an adverse effect on theoptical properties of the apparatus. Too high an operating temperatureof the cathode reduces the service life of the cathode, particularly inthe case of directly-heated filament cathodes.

A known method of adjusting the cathode temperature is based on thevisual evaluation of an image of the emissive cathode to be formed on aphosphor screen. The temperature is then adjusted such that a reasonablyuniform image is formed on the phosphor screen. This adjusting methodhas the drawback that the adjustment criterion is formed by subjectiveobservations and that a cathode image must be formed for anyreadjustment. It was found that the use of this criterion usually givesrise to an excessively high cathode temperature.

An automatically-operating filament-current adjustment for anelectron-beam apparatus comprising a thermal cathode is described inU.S. Pat. No. 3,413,517. Therein, a variation of the cathode outputcurrent, caused by an impressed filament-current variation, is comparedwith a preset reference value. The reference value is derived from anemission curve which is assumed to be the standard emission curve.Mutual differences in the emission curves for different cathodes andtemperature-dependent leakage currents from the cathode can then resultin deviations from the optimum temperature adjustment.

The invention has for its object to eliminate these drawbacks and toprovide reliable and, if desired automatically-operating,filament-temperature control. To this end, an electron-beam apparatus ofthe kind set forth according to the invention is characterized in thatthe electron-beam apparatus is provided with an interception electrodefor local interception of part of the electron beam, said interceptionelectrode forming part of the filament-current control unit.

In an electron-beam apparatus according to the invention the subjectivecriterion is replaced by an objective measuring value which is to begenerated in the electron-beam apparatus, it no longer being necessaryto use a reference value. In a preferred embodiment according to theinvention, a periodic variation is associated with the Wehneltresistance or the Wehnelt voltage, a resultant signal from theinterception screen being used for adjusting the optimum cathodetemperature.

A preferred embodiment of an electron-beam apparatus according to theinvention will be described hereinafter with reference to the drawing.In the drawing:

FIG. 1 is a diagrammatic view of an electron microscope provided with afilament-current control unit according to the invention,

FIG. 2 shows a cathode Wehnelt circuit for using a filament-currentcontrol unit with a varying Wehnelt resistance, and

FIG. 3 shows a cathode Wehnelt circuit for using a filament-currentcontrol unit with a varying Wehnelt voltage.

An electron microscope l as shown in FIG. 1 comprises, moving againstthe direction of movement of an electron beam 2 to be generated therein,a phosphor screen 3 for intercepting the electron beam, a projectionlens 4, an intermediate lens 5, a diffraction lens 6, an objective lens7, a second condensor lens 8, a first condensor lens 9, an accelerationanode 10, a Wehnelt electrode 11, and a cathode 12. In this case thecathode is formed by two interconnected filaments 13. The filaments areconnected, via filament-supply conductors 14, to a current-control unit15 which is connected to an interception electrode 17 via a coupling 16.During operation the interception electrode 17 intercepts, for example,at least 5% of the electron beam 2. The interception electrodepreferably consists of a plate having a round aperture with a diameterof approximately 300 microns. The interception electrode can also bearranged elsewhere in the electron microscope, the diameter of theaperture being adaptable to the location. The interception electrode ispreferably made of an electrically conductive material, in which case itmust be electrically insulated from the electron microscope so as to becapable of supplying an electrical output signal. The coupling 16 isthen formed by an electrical conductor. The interception electrode inanother embodiment comprises a phosphor which is to be activated by theelectron beam. Via an optical coupling 16 which is to used inconjunction therewith, a signal is obtained which is dependent of thebeam-current intensity, it being possible for said signal to beintercepted, for example, by a light-sensitive element of thefilament-current control unit 15. Provided that the entire emittedelectron beam does not reach the display screen 3, the device can alsobe constructed such that the display screen 3 or if the entire emittedelectron beam impinges upon the display screen, a portion of the displayscreen, acts as the interception electrode. An electrical or opticalsignal which is dependent of the beam-current density is then derivedfrom the display screen. The detected signal is applied to thecurrentcontrol unit 15 which controls the filament current in accordancewith the received signal.

In a preferred embodiment of a filament-current control unit as shown inFIG. 2, an adjustment signal is obtained on the interception electrodeby variation of the Wehnelt resistance. To this end, the cathode Wehneltcircuit 15', forming a part of the current control unit 15, comprises aseries of resistors 20 and a selector switch 21 for adjusting anoperating value for the Wehnelt resistance. Using a switch 22, avariation of, for example, lO to 20% of the operating value of theWehnelt resistance is effected by inserting or not inserting resistorsof a series of resistors 23 in the circuit arrangement. FIG. 2 alsoshows the two interconnected filaments 13, provided with two equalresistors 24 and 25, the Wehnelt electrode 11 and a switching point 26which is to be connected to cathode potential.

3 During generation of a signal on the interception electrode byperiodic variation of the Wehnelt resistance between the operating valueand a value which is slightly there below, the signal intensity willdecreaseif the cathode temperature is adjusted too low, and willincrease if the cathode temperature is adjusted too high. In bothsituations the filament current can be readjusted such that the signalagain obtains the initial intensity, i.e. the intensity associated withthe operating value of the Wehnelt resistance. This process cansubsequently be repeated until a Wehnelt resistance variation no longerinfluences the signal. The optimum cathode temperature is thus reached.The readjustment of the filament current can be readily effectedautomatically by providing the filament-current control unit with acircuit for generating a difference signal between the signal at theoperating value of the Wehnelt resistance and the signal at thedecreased Wehnelt resistance. The filament current is then controlled bythis difference signal, for example, via a servomechanism (not shown).

A cathode Wehnelt circuit for more practical filament-current controlaccording to the invention is shown in FIG. 3. This Figure shows theWehnelt elec trode 11, the cathode 13 with the cathode resistors 24 and25, a Wehnelt circuit 27 and a voltage source 28. The Wehnelt circuit 27generates the Wehnelt voltage, the value of which is dependent of thegeometry of the electron-beam apparatus. On the generated Wehneltvoltage a periodic, preferably sinusoidal, voltage variation issuperimposed by the voltage source 28. For example, an alternatingvoltage having a peak-to-peak value of V can be superimposed on anominal cathode Wehnelt voltage of 200 V.

Because the current density of the electron beam as a function of theWehnelt voltage is maximum exactly at the optimum cathode temperature,an alternating voltage superimposed on the Wehnelt voltage will cause asignal having a double frequency, with respect to the frequency of theimpressed alternating voltage, only at the optimum temperature setting.This is utilized in a preferred embodiment by using this frequencydoubling, after addition of a frequencysensitive detector, as anindication for the optimum temperature. An oscilloscope can be used asthe detector so that the frequency-doubling can be visually observed. lngiven cases such as, for example, in the case of an electron-beammachining apparatus where a large portion of the electron beam to beemitted by the cathode is effectively used, a phase sensitive elementcan be advantageously used for controlling the filament current. This isbecause in these cases the measuring on the edge of the electron beamproduces an insufficiently accurate impression of the brightnessvariations at the centre of the electron beam.

What is claimed is:

1. An electron-beam apparatus comprising a cathode for the thermalemission of an electron beam, A Wehnelt electrode in proximity to saidcathode for controlling said electron beam, a filament current controlunit including a variable resistance coupled between said Wehneltelectrode and said cathode for maintaining a desired emission of saidelectron beam, and an interception electrode for intercepting a portionof the electron beam said filament-current control unit being operablefor adjusting the temperature of said cathode to a value at whichminimum signals resulting from variation of said variable resistance aregenerated at said interception electrode.

2. An electron-beam apparatus as claimed in claim 1 wherein saidvariable resistance is a Wehnelt resistance and means for varying theWehnelt resistance.

3. An electron-beam apparatus as claimed in claim 2 wherein thefilament-current control unit comprises a circuit for forming adifference signal from the signals at the extreme values of the Wehneltresistance.

4. An electron-beam apparatus as claimed in claim 1 wherein thefilament-current control unit comprises a voltage generator forsuperimposing a voltage variation on a cathode-grid voltage.

5. An electron-beam apparatus as claimed in claim 4 wherein thefilament-current control unit further comprises a phase-sensitivedetector for controlling the filament-current and thereby maintainingthe temperature of said cathode, a preset phase shift with respect tothe applied signal establishing the criterion for adjustment. l

1. An electron-beam apparatus comprising a cathode for the thermalemission of an electron beam, A Wehnelt electrode in proximity to saidcathode for controlling said electron beam, a filament current controlunit including a variable resistance coupled between said Wehneltelectrode and said cathode for maintaining a desired emission of saidelectron beam, and an interception electrode for intercepting a portionof the electron beam said filament-current control unit being operablefor adjusting the temperature of said cathode to a value at whichminimum signals resulting from variation of said variable resistance aregenerated at said interception electrode.
 2. An electron-beam apparatusas claimed in claim 1 wherein said variable resistance is a Wehneltresistance and means for varying the Wehnelt resistance.
 3. Anelectron-beam apparatus as claimed in claim 2 wherein thefilament-current control unit comprises a circuit for forming adifference signal from the signals at the extreme values of the Wehneltresistance.
 4. An electron-beam apparatus as claimed in claim 1 whereinthe filament-current control unit comprises a voltage generator forsuperimposing a voltage variation on a cathode-grid voltage.
 5. Anelectron-beam apparatus as claimed in claim 4 wherein thefilament-current control unit further comprisEs a phase-sensitivedetector for controlling the filament-current and thereby maintainingthe temperature of said cathode, a preset phase shift with respect tothe applied signal establishing the criterion for adjustment.